Static random access memories (SRAMs) need not perform a refresh operation and so are lower in power consumption and faster in operation speed than dynamic random access memories (DRAMs). For this reasons, SRAMs are widely used as cash memories of computers or portable electronic products. In a case in which a computer is a notebook computer or a portable electronic product, back-up power is supplied to an SRAM at the time of power-off so as to retain data stored in the SRAM. This is one of causes of increasing power consumption at the time of power-off. Thus, it is desirable to reduce power consumption at the time of power-off in SRAMs.